The Rabshakeh mocked Judah's trust in Egypt, comparing Egypt to what?

Answer: A splintered reed

He called Egypt "that splintered reed of a staff that will pierce the hand of anyone who leans on it."

“Look now, you are trusting in Egypt, that splintered reed of a staff that will pierce the hand of anyone who leans on it. Such is Pharaoh king of Egypt to all who trust in him.”— Isaiah 36:6 (BSB)